metal casting processes

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Metal Casting Processes

AE 587: Automotive Manufacturing Processes

By

Dr. E. Orady

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Lecture Topics

Introduction Sand casting Design of mold elements Solidification of casting Fluidity Casting processes; expendable and permanent

casting processes Melting practice and Furnaces Factors affecting casting cost Casting quality Design Considerations for Casting

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Casting Processes

• DEFINITION:– The casting process is defined as the process of

melting (heating to a proper temperature above the freezing point) of a material (mostly metals) and treating it to have a proper composition, then pouring the molten material into a cavity or mold which holds it in the proper shape during solidification.

– The product of this process a casting.

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Selection of casting processes over other manufacturing processes

Casting Processes are selected for the following reasons:

» to produce complex shapes with internal cavities or hollow sections.

» to produce very large parts weighing up to 30 tons.» to utilize work piece materials that are difficult to process by

some other means.» Some casting processes are net shape; others are near net

shape» economical to use» Some casting methods are suited to mass production

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Disadvantages of Casting

Different disadvantages for different casting processes: » Limitations on mechanical properties» Poor dimensional accuracy and surface

finish for some processes; e.g., sand casting

» Safety hazards to workers due to hot molten metals

» Environmental problems

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TYPICAL EXAMPLES OF CASTING PRODUCTS

• Frames and housings of machines

• Structural parts

• Machine components

• Engine blocks

• Crank shafts

• Pistons

• Pipes

• Valves

• Rail road equipment, etc.

Some Cast Components in a Typical AutomobileSource Kalpakjian

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Categories of Casting Processes

Expendable Mold Casting processes» Mold is used one time» Molds made of sand or

Plaster or similar materials

» typical:– Sand mold– Shell mold– Expended

Polystyrene– Plaster– Investment– etc.

Permanent Mold casting processes» Mold is used over and

over to produce many castings

» Mold made of metal » Typical:

– Slush– Low pressure– Die casting– Centrifugal

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Automotive Casting Processes

Following are the commonly used casting processes in automotive applications:

Sand Casting Die casting Lost Wax casting Lost Foam casting Cosworth Casting Squeeze casting

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BASIC OPERATIONS INVOLVED IN CASTING

1. MOLDING

a) single-use molds: for small production such as sand casting, and

b) permanent molds: for large production such as die casting.

2. MELTING PROCESSES

3. POURING TECHNIQUES

4. SOLIDIFICATION PROCESS

5. SHAKEOUT AND REMOVAL OF THE PRODUCT

6. CLEANING, FINISHING AND INSPECTION

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Mold Features

Mold Terminology: Cavity Well Gating System

» Sprue or Downsprue

» Runner

» Gates Riser Parting Line Flask/Cope/Drag

FIGURE 5.10 Schematic illustration of a typical sand mold showing various features.

Manufacturing Processes for Engineering Materials, 5th ed. Kalpakjian • Schmid© 2008, Pearson EducationISBN No. 0-13-227271-7

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Sand Casting

Most widely used casting process, accounting for a significant majority of total tonnage cast

Nearly all alloys can be sand casted, including metals with high melting temperatures, such as steel, nickel, and titanium

Castings range in size from small to very large Production quantities from one to millions

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Figure 11.1 A large sand casting weighing over 680 kg (1500 lb) for an air compressor frame (photo courtesy of Elkhart Foundry).

Source: M. Groover, 2nd Ed.

Example of a Sand Cast

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Sand Casting Molds

Sand casting mold» Sand +bonding material+

Water» Sand +bonding material

Types » Green sand mold» Dry sand mold» Core sand molds» Loam molds» Shell molds» Cement bond molds

Pattern + Core = Product

A Typical Sand Mold Source: DeGarmo/Black/Kohser

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Characteristics of a Sand Mold

Strong enough to hold the metal Resist erosive action of rapidly flowing metal during

pouring Generates a minimum amount of gas when filled with

molten metal Allows gases generated to pass through Refractory enough to withstand high temperature and

strip away cleanly from the casting after cooling Core must collapse enough to permit the casting to

contract after solidification

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Properties of Molding Sand

Refractoriness» The ability of sand to withstand high temperature» It is provided by the basic nature of sand

» Common sand: Silica (SiO2), Zircon, or Olivine Cohesiveness

» The ability to retain a given shape when packed into a mold» Adding bonding materials such as: Bentonite, kaolinite, or illite

Permeability» The ability to permit gases to escape through it» Function of size and shape of sand particles, clay and moisture contents

Collapsibility» The ability to permit the metal to shrink after it solidifies and to free the

casting from surrounding mold» Obtained by adding cereals, cellulose or other organic materials that burn

out when they contact hot metal.

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Types of Molding Sand

Natural sand» Sand + natural bond clay» Used as received

Loam sand» Sand + 50% clay

(natural)» Used for large castings

Synthetic sand

» Washed sand + Binder (Bentonite) + Water

» Advantages over natural sand

– Uniform grain size

– Higher refractoriness

– Moldability with less moisture

– Require less binder

– Easier to control properties

– Less storage space

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PATTERNS

The pattern is a duplicate of the part to be cast, modified to meet the requirements of the casting process, metal being cast, and molding technique.

The pattern dimensions are the same as the final product plus a set of allowances which include :

» shrinkage allowance; for the contraction of the metal after it solidifies and cools.

» machining allowance; needed when the final product has surfaces to be machined after the casting process.

» draft allowance; a slope on the walls of the pattern to allow easy withdrew of the pattern from the mold.

» Other allowances such as distortion allowance

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PATTERNS

Pattern materials» Wood; for small quantity

» Metal; for larger quantity

» Hard Plastics; used with organically bonded sand to avoid sand stick to pattern skin

» Wax; for investment casting

» Polystyrene; used with full mold-process

Selection of pattern material is function of:» number of castings

» size and shape of the casting

» desired dimensional precision

» molding process

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Types of Patterns

Figure 11.3 Types of patterns used in sand casting:

(a) solid pattern

(b) split pattern

(c) match‑plate pattern

(d) cope and drag pattern

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CORE

A core is made when the final product has an internal cavity or hollow. The core usually has the shape of the cavity plus allowances and a support, called a core print, to hold the core in the mold. The core(in sand casting) is made from a special sand mix which is collapsible under the shrinkage stress of the casting to avoid causing cracks on the casing.

Core Box

Core made by gluing the two halves

Two Core HalvesSource:DeGarmo/Black/Kohser

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Core in Mold

Figure 11.4 (a) Core held in place in the mold cavity by chaplets, (b) possible chaplet design, (c) casting with internal cavity.

Source: M. L. Groover, 2nd Ed.

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Core Materials

Green sand Dry sand [ sand plus binder packed

in wood or metal core box] Packed sand [ made by mixing sand

with a vegetable oil or synthetic oil as binder, and water with cereal or clay to develop green strength. This process is called core-oil process. The mix is cured using hot force air at 400 to 500oF.]

CO2 sand [sand + sodium silicate (water glass)]

Shell sand [sand plus liquid thermosetting and catalyst is blown into a core box heated to around 450oF.]

Dry-sand cores for V-8 engine block

Engine Block Casting

Source: DeGarmo/ Black/ Kohser

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CORE

Characteristics » sufficient hardness and strength (after baking or hardening)

to withstand handling and forces of the molten metal» sufficient strength before hardening to permit handling» adequate permeability» collapsibility to permit shrinkage of the casting as it cools,

thereby prevent cracking and allow easy shakeout.» adequate refractoriness» smooth surface» minimum generation of gases when heated during pour

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Mold Preparation

Manual Use of jolting machines Squeezing machines Combined jolting and squeezing Automatic mold-making machines

» Match-plate machines» Vertically parted flaskless molding machine

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Manual Preparation of Sand Mold

Source: Kalpakjian

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POURING (GATING) SYSTEM

Pouring system should be designed to ensure smooth flow "laminar flow" and fill the mold at the shortest possible time before metal solidify. A typical pouring system is shown in the Figure. It consists of the following:

1. POURING BASIN

2. SPRUE :It should be designed to ensure smooth flow and avoid aspiration

3. WELL

4. RUNNERS

5. GATESPouring SystemSource: Ghosh/Mallik

ht

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POURING TIME:

The pouring time is defined as the time needed to fill or inject the molten metal into the mold. The pouring time can be estimated by applying the fluid flow laws; namely, the equation of continuity and Bernoulli's Equation

The flow velocity at any point in the pouring system (sprue , gates, and runners) is controlled by the continuity equation:

Q = A1 v1 = A2 v2 ... etc. where Q = volumetric flow rate, in.3/sec (cm3/s), A = cross sectional area,in.2 (cm2) and v= flow velocity in./sec (cm/s).

The flow velocity at the bottom of the sprue (see figure) is determined by applying Bernoulli's Equation. The velocity is

v3 = sqrt( 2. g. ht) where: g = gravitational acceleration; g=386 in./sec2 (981 cm/s2), and h3 = the height of the sprue, in. (cm)

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Pouring time VERTICAL (TOP) POURING

SYSTEM, See Figure

tf = V/ A3 v3

where V= volume of the casting including riser, in.3 (cm3) A3 = gating area in2 (cm2),

and tf= pouring time in sec. BOTTOM GATING

See Figure

where Am = the mold projection area. ( You have to do some integration to calculate the time in

bottom gating)

mttm

f hhhgA

At (

2

2

3

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Flow Characteristics

Two type of flow:» Laminar flow» Turbulent

Governing factor is Reynolds number, Re

Where: v is the velocity of the liquid, D is the diameter of the channel, and and are the density and viscosity, respectively of the fluid.

Re < 2000 laminar flow2000<Re < 20,000 is a mixture of laminar and turbulent flowRe>20,000 represent severe turbulent flow. Proper flow and design of gating system is needed to prevent

mold erosion and introduction of dross and slag inside the mold cavity

vD

Re

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Sprue Design» Tapered sprue is used to

maintain constant liquid flow through it and avoid aspiration.

» Therefore, the following relation should be satisfied

Q = A2 v2 = A3 v3

Hence,

A2/A3 = sqrt(h3/h2)

Runner and Gates Design» The ratio between:

A3:Ar:Ag = 1: 4 :4

i.e. the runner area (Ar)

Ar = 4xA3

And the total gating area(Ag) Ag = Ar = 4xA3

h2

h3

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Example 1

A mold has a top gating system with a downspure of length = 175 mm. The cross-sectional area at the bottom of the sprue is 400 mm2. The sprue leads into a horizontal runner that feeds the mold cavity, whose volume = 0.001 m3. Determine (a) the velocity of the molten metal flowing through the base of the downsprue, (b) the volume rate of flow, and (c) the time required to fill up the cavity.

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Example 2

The volume rate of flow of molten metal into the downsprue from the pouring cup is 50 in.3/sec. The length of the sprue is 8.00 in. and the cross-sectional area at the top where the pouring cup leads into the downsprue is 1.0 in.2. Determine what the area should be at the bottom of the sprue in order to avoid aspiration of the liquid metal.

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Solidification of Metals

Solidification is the transformation of molten metal back into solid state

Solidification of pure metals is different from that of alloys.

Proper solidification is the second step of producing a high quality casting.

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Pure Metal Solidification


FIGURE 5.1 (a) Temperature as a function of time for the solidification of pure metals. Note that freezing takes place at a constant temperature. (b) Density as a function of time.

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Pure Metal Solidification

Solidification is the growth of favorably oriented nuclei in the direction of heat extraction» Randomly oriented small grains form near the

mold walls » Columnar grains form towards the center of the

mold » shrink cavities (pipe) forms due to low supply of

fresh liquid

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Solidification of Eutectic Alloys

Solidification occurs at a constant temperature (like pure metal).

Eutectic cells form inside the grains The properties of the cast part are affected by:

» Cooling rate» Nucleation agents» Alloy modifications

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Solid Solution Solidification

Solidification takes place over a freezing range. Grains grow in the direction of heat extraction. Initially solidified material has lower alloying element

concentration. Solidification proceeds by dendritic form. Dendritics arms can break off and re-melt. At high cooling rate, the formed grains are smaller

and the strength is improved. Dendrites result in microporosity.

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Alloy SolidificationAlloy Solidification

Figure Source: M.P. Groover

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Alloy Solidification

FIGURE 5.6 Schematic illustration of alloy solidification and temperature distribution in the solidifying metal. Note the formation of dendrites in the semi-solid (mushy) zone.


Pouring Temperature

SuperheatStart of SolidificationEnd of SolidificationSolid Cooling

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Cast Structures

FIGURE 5.5 Schematic illustration of three cast structures of metals solidified in a square mold: (a) pure metals, with preferred texture at the cool mold wall. Note in the middle of the figure that only favorable oriented grains grow away from the mold surface; (b) solid-solution alloys; and (c) structure obtained by heterogeneous nucleation of grains.

Columnar grains oriented towards the center of the moldSmall, randomly oriented grains


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Dendritic SolidificationDendritic Solidification

Ref: Porter, et al., Phase Transformations In Metals & Alloys, Van Nostrand Reinhold, UK, 1981

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Solidification Patterns for Gray Cast Iron

FIGURE 5.7 (a) Solidification pattern for gray cast iron in a 180-mm (7-in) square casting. Note that after 11 min of cooling, dendrites reach each other, but the casting is still mushy throughout. It takes about two hours for this casting to solidify completely. (b) Solidification of carbon steels in sand and chill (metal) molds. Note the difference in solidification pattern as the carbon contents increase, Source: After H. F. Bishop and W.S. Pellini.


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Cast Structures

FIGURE 5.9 Schematic illustration of cast structures in (a) plane front, single phase, and (b) plane front, two phase. Source: After D. Apelian.

FIGURE 5.8 Schematic illustration of three basic types of cast structures: (a) columnar dendritic; (b) equiaxed dendritic; and (c) equiaxed nondendritic. Source: After D. Apelian.

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Effects of Solidification RateEffects of Solidification Rate

Faster solidification results in:» Smaller microstructural features» Smaller & more uniformly dispersed porosity and

intermetallics» Reduced grain size» Improved mechanical properties (strength, fatigue,

& ductility)

Faster solidification will not affect the morphology of inclusions

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Temperature Distribution

FIGURE 5.11 Temperature distribution at the mold wall and liquid-metal interface during solidification of metals in casting.


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Solidification Example

FIGURE 5.12 Solidified skin on a steel casting; the remaining molten metal is poured out at the times indicated in the figure. Hollow ornamental and decorative objects are made by a process called slush casting, which is based on this principle. Source: After H.F. Taylor, J. Wulff, and M.C. Flemings.


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Shrinkage

Stages» Liquid contraction during

cooling prior to solidification

» Solidification shrinkage: Contraction during the phase change from liquid to solid

» Thermal contraction of the solidified casting during cooling to room temperature.

Solidification StagesSource: Groover

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Shrinkage and Contraction DataShrinkage and Contraction Data

Volumetric Contraction

Metal

Solidification Shrinkage, %

Solid Thermal Contraction, %

Aluminum 7.0 5.6

Al Alloys 7.0 5.0

Gray cast iron 1.8 3.0

Gray cast iron, high C 0 3.0

Low C cast steel 3.0 7.2

Copper 4.5 7.5

Bronze 5.5 6.0

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HEAT EXTRACTION AND SOLIDIFICATION TIME

The solidification time (TST) of a mold can be estimated using Chvorinov's law:

TST = Cm(V/A)n

where TST = Total solidification time in minutes

n = is an exponent has a value between 1.5 to 2.0

V= casting volume in in.3 (cm3)

A= surface area of the casting in.2 (cm2).

Cm= the mold constant which depends on:

• metal characteristics ( density, specific heat, and heat fusion)

• the properties of the mold material( density, thermal conductivity, and specific heat)

• amount of superheat

» Cm can be determined experimentally for each mold and metal combination.

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Riser design

The minimum size of a riser can be determined from Chvorinov's rule; the time of solidification of the riser (TSTr) should be at least 25% longer than that for the casting (TSTc):

TSTr = 1.25 TSTc

Taking n =2

(V/A)2 r = 1.25 (V/A)2

c Calculation of the riser geometry requires that V/A be maximum, and The size of the riser should be greater than the shrinkage volume of the

casting.

Vr > Shrinkage volume of the casting

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TYPES OF RISERS

1. Top Riser:

» Characteristics:– sits on top of the casting– short feed distance

required– occupies less space in

the flask,

2. Side Riser:

» Characteristics:– located adjacent to the

mold cavity in the horizontal direction.

– longer feeding distance– occupies some of the

flask space.

Basic types of risersSource: DeGarmo/Black/ Kohser

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Riser location and feeding Distance

Risers must solidify after casting

The riser should be placed so that it is continuously feed the casting and directional solidification is maintained

Riser should be placed on the thermal center of the casting

The feeding distance should be less than or equal the following recommendations.

Max. distance between risers

Placement of risers and chill blocksSource: Ghosh and Mallik

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Riser Aids

Purpose» Promoting directional solidification» Reducing number and size of risers

Methods» External chills» Internal chills» Reducing the cooling rate of risers

– Use open risers– Use insulating sleeves around the rise– Surround the sides and top of riser with exothermic

materials that supply added heat to the riser.

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ChillsChills

Heat sinks that promote directional solidification Internal or External

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Chills

FIGURE 5.35 Various types of (a) internal and (b) external chills (dark areas at corners), used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metal, as shown in (c).


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Example 3

A cylindrical riser is to be designed for a sand mold. The length of the cylinder is to be 1.25 times its diameter. The casting is a square plate, each side = 10 in., and thickness = 1.25 in. If the metal is cast iron and Cm = 16.0 min./in2 in Chvorinov’s rule, determine the dimensions of the riser so that it will take 30% longer for the riser to solidify.

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Fluidity

Fluidity is defined as the ability of a molten metal to flow and fill the mold.

Fluidity measurement:» Length of a spiral

shape cast (Figure a)» Plate mold length

(Figure b)» Length of a fill under

vacuum

Methods of measuring fluiditySource: El-Wakil

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Factors Affecting Fluidity

Mold Design» Component dimensions, sprue, runners, riser affect fluidity to

varying degree Mold Materials and its characteristics

» The higher the thermal conductivity and the rougher the surface the lower the fluidity.

Amount of Superheat» Fluidity increases with the increase of superheat. Increase of

superheat lowers viscosity and delays solidification. Mold temperature

» Fluidity increase with increasing mold temperature Type of solidification

» Columnar is helpful» Dendritic slows down flow

Rate of pouring» Fluidity decreases with the decrease of the pouring rate.

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Shell Molding: Casting process in which the mold is a thin

shell of sand held together by thermosetting resin binder

» teps

A heated match plate is placedon the box containing sand with resin binder

Invert the box so that the sand resin fall on the hot plate and form a shell

Reposition the box to clear awayuncured sand. The mold and shell are then placed in a furnace for several minutes to complete curing.

Strip shell molds from the pattern

Assemble two halves andplace support with sand or metal shot in a box.The mold now is ready for pouring.

Source: DeGarmo/Black/Kohser

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Shell Molding

Advantages

» Provides better surface finish than ordinary sand casting (100 in or 2.5 m)

» Better dimensional accuracy, Tolerance of 0.003 to 0.005 (0.08 to 0.13 mm) are quite common.

» Economical; less labor, no further machining, etc.

» Good collapsibility of the mold help in avoiding tearing and cracking of the casting.

» The process can be completely mechanized

Disadvantages

» More expensive metal pattern, thus it can not be justified for low volume production.

» Not suitable for large size products above 25 lb.

Two halves of a shell mold pattern

Shell Mold

Final Product

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Expanded Polystyrene Process

Uses a mold of sand packed around a polystyrene foam pattern which vaporizes when molten metal is poured into mold

Other names: lost‑foam process, lost pattern process, evaporative‑foam process, and full‑mold process

Polystyrene foam pattern includes sprue, risers, gating system, and internal cores (if needed)

Mold does not have to be opened into cope and drag sections

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Expanded Polystyrene Process(Lost Foam Casting)

(1) Polystyrene patternis made and assembled

(2) The pattern is dipped in refractory slurry or sprayed byrefractory compound

(3) The pattern is placed in a metal box and supported by sand

(4) The sand is compactedby vibration

(5) The molten metal is thenpoured in the polystyrenepattern.

(6) Casting is removed andsand reclaimed

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Expanded Polystyrene Process (Lost Foam Casting)

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Expanded Polystyrene Process (Lost Foam Casting)

Advantages» Patterns need not to be removed from the mold;

– speed up the process of mold making, – no draft or parting lines are needed,– cores and risers build in the pattern

» Precision and surface finish are sufficiently good, then many machining and finishing operations could be eliminated.

» High metal utilization» Sand can be recycled» There is no limitations on the shape and size of product. » Most metal can be cast

Limitations» Pattern cost can be high for small quantities» Patterns are easily damaged or distorted because of their low

strength

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Expanded Polystyrene Process

Applications:» Mass production of castings for automobile engines

» Automated and integrated manufacturing systems are used to 1. Mold the polystyrene foam patterns and then

2. Feed them to the downstream casting operation

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Investment Casting (Lost Wax Process)

A pattern made of wax is coated with a refractory material to make mold, after which wax is melted away prior to pouring molten metal

"Investment" comes from a less familiar definition of "invest" - "to cover completely," which refers to coating of refractory material around wax pattern

It is a precision casting process - capable of producing castings of high accuracy and intricate detail

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Investment Casting (Lost Wax)

Steps:(1) Wax patterns are produced in a mold (a, and b)(2) patterns are attached to a sprue to form a tree (c)(3) The tree is then coated with a thin layer of refractory material (d).(4) The full mold (f) is formed by covering it with sufficient material to make it rigid (e).(5) the mold is then placed in an oven to melt away the wax (g)(6) The hot pattern is then placed on a container and molten metal is poured.(7) The mold is the broken away to separate casting.

Source: Kalpakjian

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Investment Casting (Lost-Wax)

Advantages» Can be used to produce casting

of high accuracy and intricate detail.

» Close dimensional control; tolerances of +/- 0.003 in. are possible

» Good surface finish» Wax can be recovered and

used» A net shape process,

machining is not required Limitations

» Costly patterns and molds» Labor costs can be high» Limited size (less than 10 lb.)

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Permanent Mold Casting Processes

Method

» Molds are made of steel or fine-grain cast iron

» Mold halves or sections are hinged so that they can open or close accurately.

» Molds are preheated at the beginning of the run to maintain uniform temperature.

» Cavity surfaces are to be coated with thin layer of refractory materials

» Cores can be used with permanent molds to form interior surfaces

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Mold preheated and coatedCores are inserted and mold closed

Pour molten metal into the mold Open mold and eject product

Finished ProductSource: Groover

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Advantages» Good surface finish» Accurate dimensions (within 0.01 in.)» Solidification can be controlled using

proper chill design» Faster cooling rate produces stronger

material than with sand casting.» Multiple cavities can often be included

in a single mold

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Disadvantages» Limited to low melting point metals

– Common metals include alloys of aluminum, magnesium, copper, lead, tin, and zinc; irons and steel can also be cast in graphite molds.

» High initial cost» Shape, size and part complexity are limitations » Low yield rate, less than 60%» Mold life is very limited; the actual mold life varies with:

– Alloy being cast– Mold material– Pouring temperature– Mold temperature– Mold configurations

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Applications of Permanent Mold Casting

Due to high mold cost, process is best suited to high volume production and can be automated accordingly

Typical parts: automotive pistons, pump bodies, and certain castings for aircraft and missiles

Metals commonly cast: aluminum, magnesium, copper‑base alloys, and cast iron

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Permanent Mold Casting :Die Casting

Metal is injected in the mold at high pressure (1000 to 50,000 psi) Pressure is maintained during solidification Combination of metal mold and pressure, fine sections and

excellent details can be achieved. Special zinc-, copper and aluminum-based alloys can be produced

with excellent properties Dies are made from hardened hot-worked steel. Dies tend to be

expensive. Dies can be designed for simple products, multiple product, or

complex products. Dies usually have water cooling passages, cores, ejectors. Dies often cost in excess of $5000 to $10,000 Die life is limited by wear, and thermal fatigue. Die casting process is limited to mass production

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Hot-Chamber Die Casting

Metal is melted in a container, and a piston injects liquid metal under high pressure into the die

High production rates - 500 parts per hour not uncommon

Applications limited to low melting‑point metals that do not chemically attack plunger and other mechanical components

Casting metals: zinc, tin, lead, and magnesium

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Hot Chamber Die Casting Machine

Components of a hot-chamber die-castingmachineSource:DeGarmo/Black/Kohser

Close chamber and let metal flow in chamber

Plunger then forces metal into die, andmaintains the pressure during part solidification

After part solidification, plunger withdrawnand die opened, and the part is then ejected using ejectors

Finished product

Source: Groover

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Cold‑Chamber Die Casting Machine

Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavity

High production but not usually as fast as hot‑chamber machines because of pouring step

Casting metals: aluminum, brass, and magnesium alloys

Advantages of hot‑chamber process favor its use on low melting‑point alloys (zinc, tin, lead)

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Cold-Chamber Die Casting Machines

Configuration of cold chamber die-casting machine

Close die and withdraw ram, then pour molten metal in the chamber Activate ram to force metal in the die

and maintain pressure until part solidify

Once part solidify, withdraw ram and activate ejection system to eject casting.

Source: Groover

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Die Casting Machine

Source: Introduction to Manufacturing Processes By John Schey, McGraw Hill, 2000.

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Hot Chamber vs. Cold ChamberHot Chamber vs. Cold Chamber

Advantages Short cycle time Better thermal control of

the process Loading a new charge

of molten metal is done automatically

The molten metal exposure to the atmosphere is reduced

Disadvantages Alloy limitations Lower injection

pressures and speed than cold chamber

Higher maintenance costs

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Advantages

Allows for casting of wider range of alloys

Higher injection pressure and speed

Lower tooling maintenance cost

Disadvantages Slower cycle times than hot

chamber process Less control of metal

temperature The charge cools prior to

injection. Molten metal exposed to

atmosphere

Cold Chamber vs. Hot ChamberCold Chamber vs. Hot Chamber

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Die Casting

Advantages» High production rate» Economical for mass production» Extremely smooth surface (40 to 100 in.) » Excellent dimensional accuracy (Typically tolerances are 0.005 in. for the

first inch and 0.002 in. for each additional inch)» Can produce thin sections up to 0.03 in.» Rapid cooling provides fine grain size and high strength.

Limitations» High initial cost» Limited to high-fluidity nonferrous metals» Part size is limited (1 oz up to 15 lb.)» Porosity may be a problem» Some scrap in sprue, runners and flash

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Multi-slide Dies

Is a variation of hot-chamber die casting

Use 4 perpendicular slides in the tool

» enables complex castings to be produced.

» In some cases, up to 6 slides can be added

This process is used primarily for casting small zinc components and is being used for casting magnesium parts

PC controllers are used to control the position of the slides

Up to 75 cycles per minute can be achieved

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Permanent Mold Casting Low Pressure Die casting

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The process is capable of producing high quality castings

Commonly cast materials Al alloys, Mg alloys, and other low melting point alloys

Sand cores can be used in the manufacture of parts with complex shapes.

Aluminum castings from 2 - 150kg can be cast, but the most common casting weight is around 10kg

High volume production is needed to justify the cost of the dies.

Permanent Mold Casting Low Pressure Die casting

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Squeeze-Casting

FIGURE 5.28 Sequence of operations in the squeeze-casting process. This process combines the advantages of casting and forging.


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Permanent Mold Casting Squeeze Casting (Melt Forging)

A variation of cold chamber pressure die casting. Results in highly refined grain structure. Used in making pistons for diesel engines. Is performed by:

» Pouring a pre-measured amount of molten metal into the die.

» Allowing the metal to cool below liquidus.» Closing the die.

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Sample of Squeeze Cast Parts

Source: http://www.entirecoupling.com/Product.htm

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Permanent Mold Casting :Vacuum Casting

The principle is the same as low-pressure die casting. The pressure inside the die is decreased by a vacuum pump and the difference of pressure forces the liquid metal to enter the die. This transfer is less turbulent than by other casting techniques so that gas inclusions can be very limited. As a consequence, this new technique is specially aimed to components which can subsequently be heat-treated.

This is an alternative to investment casting, shell mold casting, and green-sand casting

Suitable for thin-walled (0.75 mm; 0.03 in.) complex shaped with uniform properties

Can be automated.

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Vacuum-Casting Process

FIGURE 5.19 Schematic illustration of the vacuum-casting process. Note that the mold has a bottom gate. (a) before and (b) after immersion of the mold into the molten metal. Source: After R. Blackburn.


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Casting Processes Comparison

TABLE 5.8 Casting Processes, and their Advantages and Limitations.


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Properties of Die-Casting Alloys

TABLE 5.6 Properties and typical applications of common die-casting alloys.


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Additional Steps After Solidification

Trimming Removing the core Surface cleaning Inspection Repair, if required Heat treatment

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Trimming

Removal of sprues, runners, risers, parting‑line flash, fins, chaplets, and any other excess metal from the cast part

For brittle casting alloys and when cross sections are relatively small, appendages can be broken off

Otherwise, hammering, shearing, hack‑sawing, band‑sawing, abrasive wheel cutting, or various torch cutting methods are used

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Removing the Core

If cores have been used, they must be removed Most cores are bonded, and they often fall out of

casting as the binder deteriorates In some cases, they are removed by shaking casting,

either manually or mechanically In rare cases, cores are removed by chemically

dissolving bonding agent Solid cores must be hammered or pressed out

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Surface Cleaning

Removal of sand from casting surface and otherwise enhancing appearance of surface

Cleaning methods: tumbling, air‑blasting with coarse sand grit or metal shot, wire brushing, buffing, and chemical pickling

Surface cleaning is most important for sand casting» In many permanent mold processes, this step can be

avoided Defects are possible in casting, and inspection is

needed to detect their presence

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Heat Treatment

Castings are often heat treated to enhance properties Reasons for heat treating a casting:

» For subsequent processing operations such as machining

» To bring out the desired properties for the application of the part in service

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Economics of Casting

FIGURE 5.39 Economic comparison of making a part by two different casting processes. Note that because of the high cost of equipment, die casting is economical mainly for large production runs. Source: The North American Die Casting Association.


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Casting Cost Considerations

Reduced direct assembly costs

Reduced inventory

Reduced floor space

Reduced production flow, control, and inspection operations

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Factors Influencing Casting Costs

Casting Design» Size, weight, and complexity of the casting are

most important parameters Alloy Selection

» Alloying elements can be expensive (i.e. Ag)» Some alloys are more difficult to melt and pour» Higher temperatures may be needed to produce

desired fluidity» Protective environments may be needed

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Quality » How well casting meets customer’s requirements» How “repeatable” is the process.» Quality is measured by:

– Chemical and mechanical properties– “defect-free” casting– Accuracy & consistency of dimensions

» Premium quality requirements will lead to cost increase

» Producing substandard part quality leads to cost increase


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Cost of patterns/dies Cost of Tooling Production Quantity Cost of Machining Cost of Heat treatment Other Costs


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Example: Cylinder Head» Scrapped after casting: $50» Scrapped after final machining: $120» Scrapped after component assembly: $500» Replacement engine: $4000


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Process Selection Procedure

1. Identify process characteristics using product design requirements

2. Identify feasible processes (use constraints). Eliminate unfeasible ones

3. Rank feasible processes using desirable criteria (cost, lead time, No. of units, etc.)

4. Identify any additional characteristics of feasible processes

5. Select process of choice

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Process-Material Relationship

Manufacturing Process Compatible Material(s)

Sand casting F, NF

Die casting F, NF

Investment casting F, NF

Low pressure casting NF

Hot chamber die casting NF

Lost Foam casting F, NF

Cosworth casting NF

F = Ferrous, NF = Non-Ferrous

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Process-Product Relationship

Manufacturing Process Min Thicknessmm

Sand casting ~5

Die casting ~1

Investment casting 2-3

Low pressure casting 3-5

Lost Foam casting 2-3

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Other process-product relationships include Shape Capability Surface finish Dimensional tolerances Cost Etc.

Process-Product Relationship

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Casting Applications

TABLE 5.3 Typical applications for castings and casting characteristics.


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Metals for Casting

Most commercial castings are made of alloys rather than pure metals » Alloys are generally easier to cast, and properties of

product are better Casting alloys can be classified as:

» Ferrous

» Nonferrous

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Ferrous Casting Alloys: Cast Iron

Most important of all casting alloys Tonnage of cast iron castings is several times that of

all other metals combined Several types: (1) gray cast iron, (2) nodular iron, (3)

white cast iron, (4) malleable iron, and (5) alloy cast irons

Typical pouring temperatures 1400C (2500F), depending on composition

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Properties & Applications of Cast Iron

TABLE 5.4 Properties and typical applications of cast irons.


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Cast Irons

Gray cast iron : » 2.5% to 4% C and 1% to 3% Si» Has graphite flakes

Ductile Iron:» Has similar C and Si content as gray cast iron but with

graphite spheroids White cast iron

» 2% to 3.3% C and 0.7% to 2% Si» Produced by rapid cooling thus has cementite rather than

flakes Malleable iron

» Heat treated white cast iron to get carbon out of cementite to form graphite

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Microstructure for Cast Irons

FIGURE 5.14 Microstructure for cast irons. (a) ferritic gray iron with graphite flakes; (b) ferritic nodular iron, (ductile iron) with graphite in nodular form; and (c) ferritic malleable iron. This cast iron solidified as white cast iron, with the carbon present as cementite (Fe3C), and was heat treated to graphitize the carbon.


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Ferrous Casting Alloys: Steel

The mechanical properties of steel make it an attractive engineering material

The capability to create complex geometries makes casting an attractive shaping process

Difficulties when casting steel:» Pouring temperature of steel is higher than for most

other casting metals 1650C (3000F)

» At such temperatures, steel readily oxidizes, so molten metal must be isolated from air

» Molten steel has relatively poor fluidity

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Nonferrous Casting Alloys: Aluminum

Generally considered to be very castable Pouring temperatures low due to low melting

temperature of aluminum » Tm = 660C (1220F)

Properties: » Light weight

» Range of strength properties by heat treatment

» Easy to machine

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Nonferrous Casting Alloys: Copper Alloys

Includes bronze, brass, and aluminum bronze Properties:

» Corrosion resistance

» Attractive appearance

» Good bearing qualities Limitation: high cost of copper Applications: pipe fittings, marine propeller blades,

pump components, ornamental jewelry

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Nonferrous Alloys

TABLE 5.5 Typical properties of nonferrous casting alloys.


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Aluminum Casting

Aluminum is cast using» Sand casting.» Permanent Mold (gravity feed) casting.» Cold chamber pressure die casting.» Cosworth casting.» Low pressure die casting.

Steel dies are used in die casting of Al

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Advantages of Al Alloys for Casting

Low melting temperature. Negligible solubility of all gases (except hydrogen) in

molten Al. Good surface finish of the cast product. Good fluidity. Better creep properties than wrought Al alloys.

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Hydrogen Solubility in Aluminum

FIGURE 5.36 Solubility of hydrogen in aluminum. Note the sharp decrease in solubility as the molten metal begins to solidify.


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Problem with Al Castings

Variability in mechanical properties. Shrinkage in the amount of (3.5%- 8.5%). Mechanical properties are inferior to wrought Al

products.

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Advantages of Mg Die Casting

High fluidity in most alloys Lower volumetric specific heat than Al and Zn Low density Low solubility of Fe in liquid Mg Good machineability.

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Post-Casting Operations for Mg

Trimming Heat treating Machining Surface treatment Forming Joining

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Characteristics of Zinc Die Castings

Fast cooling rate, thus fine grains

Castings solidify from mold walls to center creating fine grains with low porosity in walls, and larger more porous grains in the core

Wall

Core

http://www.dezign.org/zinc

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Mold made of steel or graphite

U-shaped riser is often used with gravity die casting

Thicker gates are also used with gravity die casting

Characteristics of Zinc Die Castings

http://www.dezign.org/zinc

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Characteristics of Casting

TABLE 5.2 General characteristics of casting processes.


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Furnaces for Casting Processes

Furnaces most commonly used in foundries:» Cupolas

» Direct fuel‑fired furnaces

» Crucible furnaces

» Electric‑arc furnaces

» Induction furnaces

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Cupolas

Vertical cylindrical furnace equipped with tapping spout near base

Used only for cast irons» Although other furnaces are

also used, the largest tonnage of cast iron is melted in cupolas

The "charge," consisting of iron, coke, flux, and possible alloying elements, is loaded through a charging door located less than halfway up height of cupola

Cupola furnace used in melting cast ironSource: Groover

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Direct Fuel‑Fired Furnaces

Small open‑hearth in which charge is heated by natural gas fuel burners located on side of furnace

Furnace roof assists heating action by reflecting flame down against charge

At bottom of hearth is a tap hole to release molten metal

Generally used for nonferrous metals such as copper‑base alloys and aluminum

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Crucible Furnaces

Metal is melted without direct contact with burning fuel mixture

Sometimes called indirect fuel‑fired furnaces Container (crucible) is made of refractory material or

high‑temperature steel alloy Used for nonferrous metals such as bronze, brass,

and alloys of zinc and aluminum

Three types used in foundries: (a) lift‑out type, (b) stationary, (c) tilting

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Crucible Furnaces

Figure 11.19 Three types of crucible furnaces: (a) lift‑out crucible, (b) stationary pot, from which molten metal must be ladled, and (c) tilting-pot furnace.

Source: Groover

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Electric‑Arc Furnaces

Charge is melted by heat generated from an electric arc

High power consumption, but electric‑arc furnaces can be designed for high melting capacity

Used primarily for melting steel

Figure 6.9 Electric arc furnace

for steelmakingSource: Groover

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Induction Furnaces

Uses alternating current passing through a coil to develop magnetic field in metal

Induced current causes rapid heating and melting

Electromagnetic force field also causes mixing action in liquid metal

Since metal does not contact heating elements, environment can be closely controlled to produce molten metals of high quality and purity

Melting steel, cast iron, and aluminum alloys are common applications in foundry work

Figure 11.20 Induction furnaceSource: Groover

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Ladles

Moving molten metal from melting furnace to mold is sometimes done using crucibles

More often, transfer is accomplished by ladles

Figure 11.21 Two common types of ladles: (a) crane ladle, and (b) two‑man ladle. Source: Groover

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Casting Quality

There are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the product

The defects can be classified as follows:» General defects common to all casting processes

» Defects related to sand casting process

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A casting that has solidified before completely filling mold cavity

Figure 11.22 Some common defects in castings: (a) misrun

General Defects: Misrun

Source: Groover

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Two portions of metal flow together but there is a lack of fusion due to premature freezing

Figure 11.22 Some common defects in castings: (b) cold shut

General Defects: Cold Shut

Source: Groover

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Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze

Figure 11.22 Some common defects in castings: (d) shrinkage cavity

General Defects: Shrinkage Cavity

Source: Groover

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Casting DefectCasting Defect

Micro Defects Gas Porosity Microshrinkage Porosity or

microporosity Inclusions

Inclusions

Courtesy of Dr. J. Boileau

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Elimination of Porosity in Castings

FIGURE 5.37 (a) Suggested design modifications to avoid defects in castings. Note that sharp corners are avoided to reduce stress concentrations; (b, c, d) examples of designs showing the importance of maintaining uniform cross-sections in castings to avoid hot spots and shrinkage cavities.


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Balloon‑shaped gas cavity caused by release of mold gases during pouring

Figure 11.23 Common defects in sand castings: (a) sand blow

Sand Casting Defects: Sand Blow

Source: Groover

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Formation of many small gas cavities at or slightly below surface of casting

Figure 11.23 Common defects in sand castings: (b) pin holes

Sand Casting Defects: Pin Holes

Source: Groover

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When fluidity of liquid metal is high, it may penetrate into sand mold or core, causing casting surface to consist of a mixture of sand grains and metal

Figure 11.23 Common defects in sand castings: (e) penetration

Sand Casting Defects: Penetration

Source: Groover

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A step in cast product at parting line caused by sidewise relative displacement of cope and drag

Figure 11.23 Common defects in sand castings: (f) mold shift

Sand Casting Defects: Mold Shift

Source: Groover

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Foundry Inspection Methods

Visual inspection to detect obvious defects such as misruns, cold shuts, and severe surface flaws

Dimensional measurements to insure that tolerances have been met

Metallurgical, chemical, physical, and other tests concerned with quality of cast metal

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Design Considerations for Casting

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Design for Casting

FIGURE 5.38 Suggested design modifications to avoid defects in castings. Source: Courtesy of The North American Die Casting Association.


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Product Design Considerations

Corners on the casting:» Sharp corners and angles should be avoided, since

they are sources of stress concentrations and may cause hot tearing and cracks

» Generous fillets should be designed on inside corners and sharp edges should be blended

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Draft Guidelines:» In expendable mold casting, draft facilitates removal of

pattern from mold – Draft = 1 for sand casting

» In permanent mold casting, purpose is to aid in removal of the part from the mold

– Draft = 2 to 3 for permanent mold processes

» Similar tapers should be allowed if solid cores are used

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Core Elimination

Minor changes in part design can reduce need for coring

Figure 11.25 Design change to eliminate the need for using a core: (a) original design, and (b) redesign.

Source: Groover

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Dimensional Tolerances and Surface Finish:» Significant differences in dimensional accuracies

and finishes can be achieved in castings, depending on process:

– Poor dimensional accuracies and finish for sand casting

– Good dimensional accuracies and finish for die casting and investment casting

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Machining Allowances:» Almost all sand castings must be machined to achieve

the required dimensions and part features

» Additional material, called the machining allowance, is left on the casting in those surfaces where machining is necessary

» Typical machining allowances for sand castings are around 1.5 and 3 mm (1/16 and 1/4 in)

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Shrinkage behavior of Various Alloys

The defects caused by shrinkage vary with the type of alloy. This can be explained with the adjoining figure 1. All alloys exhibit a large shrinkage volume.

» (a) Directionally solidifying alloys cause large shrinkage voids.

» (b) An eutectic type alloy causes shrinkage depression.

» (c) An Equiaxed solidifying alloy produces shrinkage in the form of small voids and dispersed shrinkage. Showcasing

various shrinkage defectsfor different alloys.

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Solidification and Shrinkage

The figures above showcase various solidifications and their respective shrinkages. The shrinkage associated with each type of alloy changes with the riser and plate design. Consider the Fig 2(a) in this the alloy is a directionally solidifying type and by adding a taper to the plate the shrinkage has been forced from plate to the riser. In the case of eutectic alloy shown in Fig 2(b) there is shrinkage associated in the form of a depression in the riser. While in equiaxed alloy as shown in Fig 2(c), no change in the plate taper results in less shrinkage. In this case sections must be frozen at the same rate if possible.

Figure 2a: DirectionallySolidifying Alloy.

Figure 2b: Eutectic Alloy. Figure 2c: Equiaxed Alloy.

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Design Strategies to Counteract Shrinkage

Provision of a riser near the heavier section in a casting results in reduction of shrinkage.

Proper design of casting whereby lighter section follows a heavier section resulting in less shrinkage.

Proper design of casting helps in the reduction of shrinkage defects.

Source: Schrader & Elshennawy

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Design Strategies to Counteract Shrinkage

Replacing sharp corners and angles with fillets (Fig.4) results in less defects associated with shrinkage along with a reduction in stresses caused due to thermal stress concentrations.

Providing fillets and taper helps in Reduction of thermal stresses near joints.


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Strategies to Avoid Defects During Cooling

Mechanical stresses are induced in a casting on cooling. The solidification process in metals always proceeds from the mold face to the center of the casting.

Sharp corners and angles cause higher stresses and hence rounding of corners is suggested in the Figure Rounding of corners necessary for

reduction in stresses.


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Strategies to Avoid Defects during Cooling

The design engineer must take into consideration the cooling curves for various junction designs. The best casting design would entail bringing the minimum number of sections and also by avoiding acute angles along with large fillets.

Cooling curves can be consulted in designing junctions in castings.


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Slag and Dross Formation

Slag/Dross – These are synonyms meaning “refuse from melting of metals”. Although slag is usually referred with higher melting point metals and dross with lower melting point metals.

Various metals have varying tendencies for the formation of slag/dross. This makes it critical in choosing the right alloy for the casting. Not necessarily choosing the alloy which causes less slag/dross but by choosing the alloy keeping in mind the application.

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Pouring Temperature

Pouring temperature becomes a critical parameter in casting design due to the extremely high temperatures associated with molten metal.

The designer must hence take into consideration problems associated with thermal degradation of the mold and formation of hot spots.

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Metal Alloys and Their Approximate Pouring Temperatures

Table 2 courtesy “Tool and Manufacturing Engineers Handbook” provides the design engineer with approximate pouring temperatures which would result in the least amount of thermal abuse.

The pouring temperatures would also be affected with the type of mold material, eg: Only titanium alloys are poured in graphite molds.

Table 2: Table gives recommended approximate poring temperatures.

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Section Thicknesses

Due to variable cooling rates sections designed should be as uniform in thickness as possible. Non uniform sections would cause defects in the casting due to the variable cooling rates. Chills can be provided to counteract this phenomenon

Uniform thickness of section for the lugs is recommended.


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Correct proportioning of inner wall dimensions

Cooling rates of inner portions are much slower compared to outer surfaces. This makes it necessary to avoid as far as possible sharp angles and corners.

A good rule of thumb is to reduce the inner sections to 9/10th the thickness of the outer walls.

Reduction of the inner section to 9/10th of outer walls.


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Correct proportioning of inner wall dimensions

For economy purposes the radius of the inner cylinder should be bigger than the wall thickness. As in in the Figure since the inner radius is much smaller it is advisable to cast it as a solid and to then drill the required hole.

Inner cylinder should be bigger than the casting wall thickness.


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Rib Design Principles

Since ribs are designed to increase stiffness and for weight reduction. So if the ribs are designed with less depth or are widely spaced they become redundant.

Thickness of the ribs should be 80% of casting thickness and the ribs should be rounded at the edges and filleted correctly.

Figures show the Do’s and Don’t’s of Rib design in castings.


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Rib Design

Design of complex ribbing should be avoided where necessary due to simplification in casting process on the whole.

If the casting wall itself can provide the necessary stiffness then omission of ribs is recommended.

Complex Rib design should be avoided if possible.


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Bosses, Lugs and Pads

Since bosses and pads increase the metal thickness this entail results in hot spots in the casting requiring the presence of chills etc. Hence it is recommended to not use these elements where possible.

Best design practices for bosses and pads.


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Bosses, Lugs and Pads

As is the case with junction design the design of bosses should be such that they seamlessly mate with the casting with the help of proper tapers and allowances as in the Figure.

Table 3 gives an approximate reference guide for the heights of bosses.

Table 3: Guide to designing bosses.

Recommended design for bosses and pads.


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Redundancies

By providing a recess in the above casting. Cost becomes a factor due to the need for a core. This can be reduced if the casting can be done solid without the recess. So the design engineer must work closely in minimizing the complexities involved in the casting.

A projection increases the cost of making a casting


metal casting processes

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